Los Angeles police served the leader of the biggest union at the city Department of Water and Power with a subpoena on Friday for records on the spending of millions of dollars in ratepayer money by two secretive nonprofits that he co-manages.

At the same time, Mayor Eric Garcetti took exception to union leader Brian D’Arcy’s reported threat that he would sue the DWP’s general manager personally if he disclosed the nonprofits’ spending records.

The mayor and D’Arcy have been at war politically since affiliates of D’Arcy’s union, Local 18 of the International Brotherhood of Electrical Workers, ran a brutal independent advertising campaign against Garcetti in last year’s election.

The conflict was part of what led DWP General Manager Ron Nichols to resign Thursday, according to people familiar with the matter. Nichols, they said, did not want to be in the center of the conflict, and Garcetti wanted a loyalist in the job to lead a bold challenge to the union’s work rules.

Asked on Friday whether Nichols could have been more aggressive with the union, Garcetti responded: “I think that’s a fair observation.”

Garcetti has promised sweeping reform of the unpopular utility, whose high salaries and ever-increasing rates have long irked many L.A. voters. Part of Garcetti’s effort involves forcing disclosure of the spending records of the two nonprofits, the Joint Training Institute and the Joint Safety Institute.

“I’m looking for someone who will be aggressive about these things,” Garcetti told reporters at a news conference in Pico-Union.

The institutes, run jointly by the union and the DWP, have spent more than $40 million in ratepayer money over the last decade. D’Arcy has refused to disclose any records on the nonprofit trusts, and Nichols has been unable to account for their spending.

The trusts were established to improve relations between DWP management and the union.

City Controller Ron Galperin is conducting an audit of the institutes, but D’Arcy has refused to cooperate. On Friday, police served D’Arcy with a subpoena at his IBEW office. It orders D’Arcy to testify under oath and provide the financial records on Jan. 23.

D’Arcy could not be reached for comment.

On Thursday, DWP board President Mel Levine told The Times that D'Arcy had threatened to sue Nichols personally if he made the records public.